As long as it's came from a Spec 10.5 then you should be able to run the rotor. Just be careful, there are Mod 10.5's and Spec 10.5's.
Mod Rotor #987, #957, or #958.
Spec Rotors are #954,#955,#956.
If legality isn't an issue, then go for the 13.0. Torque is king, especially in 21.5 racing.
Edit - Just saw your post now.